With the recent tendencies for electronic equipment to have a smaller size and higher performance, it has been demanded for semiconductor devices constituting electronic equipment and multilayer printed wiring boards for mounting the devices to have a reduced size, high performance and high reliability. To meet these demands pin insertion mounting is being displaced by surface mounting, and, in recent years, a surface mount technology called bare chip mount has been under study, in which bare semiconductor elements are directly mounted on a printed board. The chips used in bare chip mount, which are not packaged, are usually encapsulated with an encapsulating resin called an underfilling material. The main object of the underfilling material is to disperse the stress produced by the difference in coefficient of thermal expansion between a circuit board and a semiconductor element. Therefore the underfilling material must keep a certain modulus of elasticity even in high temperature. From this viewpoint, thermosetting resins are usually used as an underfilling material.
The underfilling material (usually a thermosetting resin) is generally cast in the gap under a semiconductor element bonded to a circuit board, followed by post curing. However, the conventional technique involves various problems such as a low yield, a long post curing time, and poor repairability. It has therefore been demanded to develop a circuit board on which semiconductor elements can be mounted easily with high reliability.